In order to ensure that you're not throwing your money down the drain, when it comes to your marketing campaigns, it's important to track your advertising.
Always make sure to carefully track the results of your direct mail, pay-per-click, ezine, banner ads, etc.
These are measurable by the amount of responses you get per dollar spent, and you can quickly analyze results to determine whether or not to continue advertising - or if you need to make adjustments to your ads.
It may be necessary to tweak your copywriting during a split-test of different ads to make sure you're achieving the desired response.
You are split-testing your ads aren't you?
If you're not, you're leaving a ton of money on the table.
In case you don't know what split-testing is, it's basically creating multiple versions of your ads to see what version converts more visitors into sales.
Google Analytics allows you to accurately track your advertising for free.